Resonant Levels and Optimized Thermoelectric Performance in Cr‐Filled ZrCoBi 0.8 Sn 0.2

ABSTRACT Vacancy‐filling strategy not only expands the Heusler family but also efficiently optimizes their thermoelectric performance. In this work, Cr atoms with 6 valence electrons are filled into the vacant 4d site of ZrCoBi 0.8 Sn 0.2 , preserving its semiconducting behavior. A small amount of Cr filling does not alter the carrier concentration of ZrCoCr x Bi 0.8 Sn 0.2 but introduces resonant levels within the bandgap, significantly enhancing the density of states effective mass from 13 m e ( x = 0) to 20 m e ( x = 0.04). As a result, the room‐temperature Seebeck coefficient increases by ∼43% in the ZrCoCr 0.04 Bi 0.8 Sn 0.2 sample. Meanwhile, partial filling of the 4d site induces mass and strain fluctuations, substantially reducing the lattice thermal conductivity. At 950 K, the lattice thermal conductivity of ZrCoCr 0.02 Bi 0.8 Sn 0.2 decreases to 1.65 W m −1 K −1 , among the lowest values reported to date for p‐type ZrCoBi‐ and ZrCoSb‐based half‐Heusler alloys. Consequently, a thermoelectric figure of merit zT of ∼1.0 is achieved in p‐type ZrCoCr 0.02 Bi 0.8 Sn 0.2 at 950 K, representing an improvement of ∼47% relative to the pristine ZrCoBi 0.8 Sn 0.2 . This work not only illustrates the promise of the vacancy‐filling strategy for advancing Heusler thermoelectrics, but also offers valuable insight into the regulation of material properties through vacancy filling.